In the operation of a lithium ion battery, the anode takes up lithium ions from the cathode when the battery is being charged and releases those ions back to the cathode when the battery is being discharged. One important parameter of the anode material is its capacity for retaining lithium ions, since this will directly impact the amount of charge which a given battery system can retain. Another important parameter is reversibility, that is to say the number of times the material can take up and release lithium ions without degradation or significant loss of capacity. This parameter will directly influence the service life of the battery system.
Current lithium ion battery systems generally employ a carbonaceous anode due to the fact that it has very high reversibility and is very safe. One problem with carbon materials is that their lithium ion capacity is only moderately high; hence, relatively large amounts of anode material must be employed in a given battery system. Silicon is capable of alloying with relatively large amounts of lithium and hence has a number of advantages as an anode material for lithium ion batteries. Typical carbon based anodes have discharge capacities of approximately 372 mAh/g while silicon has a theoretical capacity of 4200 mAh/g. However, silicon undergoes a relatively large volume change when lithium is incorporated therein and this volume change is very disadvantageous in most battery systems, since it can cause a loss of capacity, decrease cycle life, and cause mechanical damage to the battery structure.
Because of silicon's potential advantages as an anode in a lithium ion battery system, the prior art has made attempts to overcome problems of mechanical damage and reversibility so as to allow it to be used in battery systems. Toward that end, the prior art has sought to utilize alloys, composites, and other complex structures of silicon and other materials in anodes of lithium ion batteries. Some such approaches are disclosed in U.S. Patent Application Publications 2007/0077490, 2007/0190413 and 2005/0282070; U.S. Pat. No. 7,316,792; and published PCT Application WO 2007/015910.
Despite the various efforts, the prior art has not been able to successfully utilize silicon based materials to prepare high capacity anode structures for lithium ion batteries which manifest high cycle lives. As will be explained in detail hereinbelow, the present invention provides a composite, silicon based electrode material which is capable of alloying with large amounts of lithium ions and which retains this ability through a large number of charge/discharge cycles. The anode materials of the present invention allow for the manufacture of high capacity lithium ion batteries which have very good cycle life. These and other advantages of the present invention will be apparent from the drawings, description and discussion which follow.